1. Field of the Invention
The present invention relates to a fixing device disposed in an image forming apparatus such as an electrophotographic copying machine and a printer, and more particularly, relates to a fixing device comprising a pressure roller and a heat roller between which a recording sheet having a toner image transferred thereon is conveyed so that the toner image can be fixed to the recording sheet.
2. Description of the Prior Art
In an electrophotographic copying machine, a photosensitive drum is charged and exposed to the light reflected from an original so that an electrostatic latent image corresponding to an image on the original can be formed thereon. The electrostatic latent image is then developed using toner. The toner image formed on the photosensitive drum is transferred to a recording sheet by a transferring device, and the transferred toner image is then fixed to the recording sheet by a fixing device.
A conventional fixing device as shown in FIG. 9 generally comprises a heat roller 110 including a rotatable roller body 111 and a heater 112 mounted through the roller body 111 and a pressure roller 120 for pressing the roller body 111. The heater 112 of the heat roller 110 includes a plurality of heating portions 113 arranged at equal intervals along the axis of the heat roller 110 in such a manner that a recording-sheet contact area S on the circumferential surface of the roller body 111 which directly contacts a recording sheet can be heated uniformly in the axial direction of the heat roller 110. Therefore, the distance between one of the end faces of the heater 112 and one of the ends of the recording-sheet contact area S nearer to the above end face and the distance between the other end face of the heater 112 and the other end of the area S are equal, as is shown as a in FIG. 9. When the recording sheet is conveyed between the rollers, a toner image on the recording sheet is heated by the surface of the roller body 111 of the heat roller 110, and, at the same time, the toner image is pressed to the recording sheet by the pressure of the heat roller 110 and the pressure roller 120, so that the toner image is fixed to the recording sheet.
An electrophotographic copying machine comprising the above-described conventional fixing device is provided with an exhaust fan 90 so as to prevent a temperature rise in the machine caused by the heating of the heat roller 110. When the exhaust fan 90 is disposed on one side of the heat roller 110 as shown in FIG. 9, the air around the heat roller 110 flows in the axial direction of the heat roller 110 toward the exhaust fan 90 shown by arrow D and is exhausted outside the machine through the exhaust fan 90, without convecting inside the machine.
In the above-described structure, however, the air located upstream of the air flow with respect to the end face of the roller body 111 located upstream of the air flow, which has not been heated by the heater 112, also moves toward the exhaust fan 90 and flows along the recording-sheet contact area S, thereby cooling the surface of the roller body 111. Especially, the surface of the end portion of the roller body 111 located upstream of the air flow which is directly cooled by the unheated air flow is not likely to be heated enough to reach a predetermined temperature by the heater 112 in the roller body 111.
Further, since the heater 112 mounted through the roller body 111 is not rotatable while the roller body 111 is rotated, the unheated air may enter inside the roller body 111 from a space formed between the end face of the roller body 111 located upstream of the air flow and the heater 112 mounted through the end face. As a result, the surface of the end portion of the roller body 111 located upstream of the air flow may be further cooled.
Curves of the calorific value of the heater 112 and the temperature of the surface of the roller body 111, both measured along the axis of the roller body 111, are shown in FIG. 9. As is apparent from the figure, the surface temperature of the end portion of the recording-sheet contact area S located upstream of the air flow has not reached the predetermined temperature. In such a fixing device, the toner on the receiving sheet conveyed between the heat roller 110 and the pressure roller 120 will not be heated sufficiently. As a result, the toner may not be fixed well to the receiving sheet, or the toner may attach to the surface of the heat roller 110.
Japanese Laid-Open Utility Model Publication No. 63-54161 discloses a fixing device in which a heater in a heat roller can be shifted along the axis of the heat roller by means of a solenoid or a motor so that, when a smaller-size recording sheet is conveyed to the heat roller, the end portion of the heat roller not in touch with the recording sheet is not heated. However, in the disclosed structure, when a large-size recording sheet covering the whole width of a recording-sheet contact area is used, the end portion of the area will be cooled by the air flow generated by an exhaust fan, causing the same trouble as described above.
Japanese Laid-Open Patent Publication No. 1-285973 discloses a fixing device in which a heater in a heat roller can be shifted along the axis of the heat roller in correspondence with the number of sheets or the time to be copied. However, the disclosed fixing device is not effective for preventing the end portion of an recording-sheet contact area from being cooled by the air flow generated by an exhaust fan.
An objective of the present invention is to provide a fixing device ensuring the complete fixation of a toner image to a recording sheet by preventing the end portion of a recording-sheet contact area of a heat roller located upstream of an air flow from being cooled by the air flow inside the heat roller and along the surface thereof.
Another objective of the present invention is to provide a fixing device realizing a uniform temperature at the surface of the recording-sheet contact area of the heat roller along the axis thereof.